FastFloor.hs

{-# LANGUAGE MagicHash #-}

{- |
  This module shouldn't need to exist. It exists because
  GHC's implementation of 'floor' is currently (6.10.3)
  very slow. See the following tickets:

  <http://hackage.haskell.org/trac/ghc/ticket/1434>

  <http://hackage.haskell.org/trac/ghc/ticket/2271>
-}

module Data.Colour.FastFloor where

import GHC.Prim
import GHC.Types
import GHC.Word

{- |
  This is a special version of the regular 'floor' function.
  It works by directly calling the low-level internal GHC
  primitives, and thus is as fast as you'd expect for such
  a trivial operation.

  (The standard 'floor' function does something crazy like
  converting a @Double@ to a numerator/denominator @Integer@
  pair and then computing the integer part of the quotient as
  an @Integer@, then truncating that to a @Word8@. Which,
  obviously, is ludicrously slow.)

  Hopefully one day the need for this low-level hackery
  will disappear.
-}
fast_floor :: Double -> Word8
fast_floor (D# d#) = W8# (int2Word# (double2Int# d#))